We can also see Amazon selling Chromecast, although, it's not surprising that Chromecast doesn't qualify for Amazon Associates Program Product. I guess, no company would like to make a double loss. But all in all, it's safe to assume, Google and Amazon have come to a peace treaty now.
Why it took 4 years for Google and Amazon to fix this? Well, we don't know for sure. But, Google Stadia and the upcoming Amazon Prime sale definitely have something to do with it.